Permuting bits using bitshifts can be at least 20 times faster than using String operations (in nanos)

bit_vs_str_permutations.rs

use std::time::{Instant};

fn permute_eight_bits(ten_bit: u16) -> u8
{
    // P8[6 3 7 4 8 5 10 9]
    // P8[5 2 6 3 7 4 9 8] (-1)
    let mut permuted_byte = 0b0000_0000_0000_0000;
    
    let p_eight: [(u16,u16);8] = [(5, 0b0000_0000_0010_0000),
                                  (2, 0b0000_0000_0000_0100),
                                  (6, 0b0000_0000_0100_0000),
                                  (3, 0b0000_0000_0000_1000),
                                  (7, 0b0000_0000_1000_0000),
                                  (4, 0b0000_0000_0001_0000),
                                  (9, 0b0000_0010_0000_0000),
                                  (8, 0b0000_0001_0000_0000)];

    for x in 0..8 {
        permuted_byte = permuted_byte << 1;
        if ten_bit & (1 << p_eight[x].0) == p_eight[x].1 { 
            permuted_byte = permuted_byte | 1;
        }
    }
    permuted_byte
}

fn permute_eight_str(ten_bit: u16) -> String
{
    let init_key_str = format!("{:010b}", ten_bit);
    let mut permuted_chars: Vec<char> = Vec::new();
    let mut permuted_string = String::with_capacity(8);
    let p_eight: [usize;8] = [6,3,7,4,8,5,10,9];

    for x in 0..8 {
        permuted_chars.push(init_key_str.chars().nth(p_eight[x]-1).unwrap() as char);
    }

    for c in &permuted_chars // to avoid 'move' errors, we pass a reference
    {                        // as '&permuted_chars' and dereference '*c'
        permuted_string.push(*c);
    }
    permuted_string
}

fn main() {

    let bits = 0b0000_1111_0000_1111;
    
    //call permutation using Strings
    let mut now = Instant::now();
    let string_result = permute_eight_str(bits);
    let mut new_now = Instant::now();
    let delta = new_now.duration_since(now);
    
    //call permutation using Bitshifts
    now = Instant::now();
    let bits_result = permute_eight_bits(bits);
    new_now = Instant::now();
    let delta_two = new_now.duration_since(now);
    
    println!("str: {:?}, took: {:?}", string_result, delta);
    println!("bits: {:08b}, took: {:?}", bits_result, delta_two);
}